What does John 6:47 mean?

Repetition, especially in ancient literature, was meant to signal certainty or importance. Jesus uses the Greek word amēn here. Amen was originally an Aramaic word kept intact in Greek, Latin, and many other languages. It literally means, "faithful," or "believe," and is most simply translated as "truly." This is why the term is often used at the end of a prayer or other statement. Putting it at the end of a phrase suggests—or hopes—that what was said is true or sure. Putting the word amen at the beginning of a statement is a way of claiming that this is the speaker's own personal declaration. Jesus is claiming that His statement is absolutely true and that He has absolute, direct, personal knowledge of its truth.

Jesus has been explaining the true source of eternal life, which is belief in the One sent by God. Specifically, this is Jesus Christ Himself, who is the "Bread of Life." In the prior verse, Jesus explained that He alone has come from heaven, and so His assurance that eternal life is for those who believe can be underscored with "truly, truly."